codeception/verify 2.1 removed support for expect()->stringContainsString() so we need
to replace it with verify()->stringContainsString().
[MAILPOET-5664]
This changes how the editor is initialized. The integration initialization class
doesn't get the engine class anymore, but integration is initialized via
mailpoet_email_editor_initialized action triggered from the engine.
This allows multiple integrations to hook into the engine.
[MAILPOET-5540]
This commit adds a simple implementation of ensuring the mailpoet_email post type
has associated NewsletterEntity.
In the future we will need to add support also for saving additional data for
newsletters (subject, sending settings).
[MAILPOET-5365]
